Method of producing hexafluorophosphoric acid



l atentecl Nov.

METHOD OF PRODUCING HEXAFLUORO- PHOSPHORIC ACID Willy Lange, Cincinnati, Ohio, and Ralph Livingston, Oak Ridge, Tenn, assignors t Ozark- Mahoning Company, a corporation of Delaware No Drawing. Application September 12, 1946, Serial No. 696,620

2 Claims.

This invention relates to the production and the purification of hexafluorophosphoric acid, HPFc, and to the production of its crystalline hydrated form, hexafiuorophosphoric acid hexahydrate, HPFc.6H2O, the latter being a useful intermediate product in the purification of the acid as well as commercially valuable in itself; for brevity they will hereinafter be termed respectively hexa acid and hexahydrate.

The reaction between phosphorus pentoxide (P205) and hydrofluoric acid (HF) is known to produce under appropriate circumstances certain fluorination products, and the copending application of Willy Lange for United States Letters Patent, Serial No. 542,603, filed June 28, 1944, now Patent No. 2,408,785, for Method of production of anhydrous monofiuorophosphoric acid describes the conditions under which anhydrous monoand difluorophosphoric acids are obtained therefrom, while in our copending application for United States Letters Patent, Serial No. 478,838, filed March 11, 1943, now Patent No. 2,408,784 for Anhydrous monofluorophosphoric acid and meth- 0d of producing it, we have described the results of a certain reaction between metaphosphoric and hydrofluoric acids in the. preparation of monofiuorophosphoric acid.

It has heretofore been suggested that reaction of P205 with 40% aqueous HF may produce the hexa acid. The experiment, according to the suggestion in the technical literature, namely, reaction of 90 grams of P205 with 90 grams of 40% HF, resulted in a liquid yielding with nitron a precipitate weighing 2.25 grams. This precipitate indicated in the said liquid reaction product the presence of only 0.398% hexa acid which was mixed with the very large amount, namely 69%, of other phosphorus-containing acids. Furthermore, this very low concentration of HPFG could neither be increased through the use of known concentration methods nor could any separation of the hexa acid as such, from the other phosphorus-containing acids be accomplished. Again, the reaction product, after heating under closed-container conditions, i. e. no gaseous constituents escaping, and subsequent cooling, produced no precipitate with nitron, indicating that the hexa acid, initially present, was thermodynamically unstable in the system and could not have been produced in an equilibrium reaction, a conclusion we have confirmed by extensive experiments.

In our research in connection with introduc- .ing fluorine into phosphorus-containing com- .pounds we have found, however, that under suitable conditions solutions can be produced which contain appreciable and large quantities of the hexa acid, HPFs, in thermodynamically stable form. This acid apparently cannot be isolated in anhydrous form, a conclusion also confirmed by our experiments and consistent with the complex nature of the HPFs which contains pentavalent phosphorus, but solutions produced in accordance with our invention provide a source from which the hexahydrate in pure crystalline form may be isolated and/ or recovered, and the invention accordingly contemplates production of this novel and useful composition which is a form of pure hexa acid in the presence of its water of crystallization.

Moreover, while hexa acid as such appears incapable of existence in the anhydrouspure state, we produce aqueous products containing it in ionized form in concentrations which render it available for practical use.

It is therefore a principal object of our invention to provide a method for producing and purifying products containing in commercially useful concentrations hexa acid responding to the formula HPFs. A further object is to render available for practical use the crystallized hexahydrate as a composition of matter responding to the formula HPF6.6H2O.

ther objects, purposes and advantages Of the invention will hereafter appear or be understood from the following description of certain ways we have devised for carrying it out and attaining its objects.

Essentially, our method of producing hexa acid consists in mixing any one or a mixture of more than one of a group 0f compounds of pentavalent phosphorus, including orthophosphoric acid (H3PO4) and its fiuorination and dehydration products hereinafter more fully described, with HF in such proportions and under such conditions that an equilibrium is established with hexa acid as a reaction product. Such equilibrium exists when there is no noticeable loss of hexa acid in the said reaction product when the latter is heated under conditions which prevent dissipation of any of its components by evaporation and then cooled to the original temperature (approximately room temperature or lower) that is, if the HPFG present hydrolyzes to a certain extent on such heating it is reformed in substantially like quantity on cooling; if the I-lPFs has not been formed in an equilibrium reaction, as herein defined, any HPFs present in the reaction product is thermodynamically unstable 3 and disappears on such heating and subsequent cooling.

I'I'SPO4 and all members of the group known as fluorination products of H3Po4, namely,

Monofluorophosphoric acid (HzPOsF) Difluorophosphoric acid (I-IPOzFz) Phosphorus oxytrifiuoride (POFs) Phosphorus pentafluoride (PF5) and all compounds or mixtures of compounds obtained by molecular dehydration-of HsPOi, 'or by hydration of P205, or by reaction of molecular dehydration products of Hz P04 with P205 and its hydration products maybeused for the production of hexa acid in accordance with our method; in addition to HePOr these include compounds such as:

hydrolyzedwith the result that H3PO4 is the final and only ultimate product of hydrolysis. Orthophosphoric acid. is also the final product of hydrolysis of the above mentioned fluorine derivatives of H3P04. Some of these condensed phosphoric acids are not chemical individuals but must be considered as mixtures. This fact, however, does not impair their usefulness in the process of our "invention as any mixture consisting of condensation or dehydration products of H3P04 may be used successfully.

In reacting any of the said compounds, and

particularly the known dehydration and/or fluorination products Of'HsPOL or mixtures of any of or allthese' compounds, with HF, whether anhydrous or in aqueous solution, there should be presentin the reaction, or produced thereby, sufficientiwater to providea medium for electrolytic dissociation of HPFs-and the formation of PF6 and H5O -ions; under these conditions and in accordance with our invention the equilibrium which is established is shifted by heating under closed-container 'conditions, although the original composition of themeaction product is reestablished on cooling to the'original temperature, that is to room temperature or lower.

The theoretically required quantity of fluorine necessary for the complete transformation of the phosphorus compound into HPFG is available when'the materials are present in a ratio corresponding to 6 ato1ns of fluorine per atom of phosphorus.

In carrying out theseveral reactions contemplated by our invention, consideration must be given to the fact that-the equilibrium constants are not the same for allpossible concentrations of the starting'components. we have noted that the equilibrium constant increases with increasing' HF concentration in the starting mixture and decreases with the concentration of water present at the outset. Thus, over a wide range of concentrations, the .equilibria deviate from the law of mass action. As a consequence, oertain limits for the ratios of the starting ingredients supplied must be observed so that HPFG may be produced in equilibrium. When these limits are approached very small quantities of the acid are formed. Beyond these limits, either is thermodynamically unstable, is not in equilibrium in the reaction product, and disappears .on heating the product.

With increasing concentration of HF, still within the limits of the appropriate HPEP ratio, substantially increased yields of HPFs are obtainedj with the maximum when anhydrous HF is used, although as has been noted some Water must-be either supplied with the starting ingredients, as in the case of PF5, or produced in the reaction in the cases of all the other P compounds mentioned to afford an ionization medium for HPFB. Water as an added ingredient is necessary when 131% and'anhydrous HF'are used, since none is-produced from these'compounds and hence no I-IPFe can be formed. With a small amount of water present, however, reaction between PF5 and 'HF proceeds satisfactorily to produce HPFG while the water resulting from the reaction of anhydrous HF with the other P compounds which have been mentioned as starting ingredients is adequate or more than adequate to meet ionization requirements of the HPFe produced.

The several reactions contemplated by the invention do not require temperature control and may be carried out above room temperature if a closed system is employed so that no component of the reaction mixture can escape in vapor form.

The following equilibrium equations set forth assumed reactions with HF of several of the most common of the known compounds in the family or group 'of pentavalent phosphorus compounds suitable for use in accordance with the invention, indicating certain quantities of water are produced in anhydrous systems from all except PF5; these quantities are ample or more than ample for the required ionization of the HPFs produced in each case, and intermediate reaction products are not noticed in the equations as they preferably do not appear in substantial quantities in the final equilibria:

tially lower fiuorination, products including .HzPOsF, I-lPOzFz, POF3 orPFs, which then react further with, additional -I-IF-to produce successively higher fiuorination products; the results in such cases are the ultimate production of HPFs substantially in accordance with Equations 5, 6, '7 and 8. a

In the reaction of I-IaPOi with HF under the conditions of our invention, substantially all the intermediate equilibria are established in succession in a swift sequence. If the ratio HFIHBPO4 in the starting mixture is comparatively low, the presence of H3PO4, H2PO3F and HPOzFz as well as HPFs, in the final equilibrium product can be shown by analytical means. Obviously, therefore, any of the fluorinated intermediates between H3PO4, and HPFG may be used in the production of the hexa acid solutions. If the ratio of anhydrous HFzl-lzPOi is high, a large part of the H3PO4 is transformed into HPFe and correspondingly less of any of the intermediates remains in the equilibrium product. I

The HPFs is formed in an equilibrium which is shifted, i. e. forced more and more to the left side of the equation, by water in excess of minimum requirements, and hence the highest concentrations of HPFs are obtained in the product when such excess is kept at a minimum. Thus with a given ratio of anhydrous HF or of the required concentration of aqueous HF, Reactions 7 and 8, in which POF and PFS are used, yield the highest concentrations of I-IPFe since these reactions produce the smallest quantities of water or no water; they are followed in turn by HPOzFz, Reaction 6, and I-IPOs, Reaction 2, and so on, with the lowest concentration from H3PO4, as its Reaction 3 produces 4 moles H2O with each mole HPFs. However, if some of the water, which would otherwise be formed during the establishing of the equilibrium in a mixture of HSPO4 and HF, is removed before the mixture is prepared, as by molecular dehydration of water-free H3PO4 to so-called condensed phosphoric acids or to the highest stage of molecular dehydration, namely, to P205, then the concentration of HPFs will be accordingly higher than in the product obtained from H3PO4.

Specifically, the mechanism of the reactions between molecular dehydration products of H3PO4 and aqueous HF is, to a great degree, unknown, but these dehydration products do not become components of the final equilibrium. In fact, H3PO4 is the only fluorine-free phosphorus compound found after equilibrium has been established, suggesting that all its molecular dehydration products which are not transformed into fiuorinated compounds are transformed into H3PO4 which then becomes a true component of the equilibrium to a greater or less degree depending on the concentration of the aqueous HF employed. The mechanism of the reaction when anhydrous HF is used is known to some extent; thus HPO2F2, POFs, PFs and HPFe are successively produced. Moreover, to obtain the maximum transformation of a P compound into IIPFG in the reaction.

product, some excess of HF is necessary over the theoretically required amount as indicated in the 6 gredients or of the HF, may be added to the reaction, or may result therefrom, but the amount present at the beginning of the reaction should be within certain limits as hereinafter more fully described.

While it is desirable for maximum production of HPFs that at least sufficient HF be present or supplied to satisfy the relation 6 F atoms for each P atom, if enough HF is used to satisfy the minimum requirements of our invention within the said limits as to the amount of water present, then at least some HPFs Will be formed in an equilibrium reaction.

I Our invention thus contemplates broadly the production of hexa acid in a reaction which leads to an equilibrium and in which any member of the H3PO4 family as herein described may be reacted with HF; the more important members of this family are represented by chemical formula in the following list: P205, I-IPOs, H4P2O1, H3PO4, HzPO-aF, HPO2F2, POFs and PFs, in which H3PO4 is the core of the family and is the substance into which each member of the group is transformed on complete hydrolysis. In general, subject to the maintenance of conditions which have been stated, the principal reactions taking place successively in mixtures of HF and certain typical members of the H3PO4 family may be represented by the following scheme:

Inasmuch as water is formed along with the formation of HPFS in the above equilibrium reactions, water present at the beginning of the reaction prevents the equilibrium from being established far to the right side, and hence water-free members of the H3P04 group, except PF5, when reacted with anhydrous HF generally give the most favorable equilibrium conditions for the formation of HPFe.

While we have observed that some equilibrium formation of HPFG occurs when hydrofluoric acid, especially at high concentrations or in anhydrous state, is supplied in quantity insufficient to satisfy the ratio 6 atoms F 1 atom P, it is evident that under such conditions there is in effect an excess of the P-containing compound and hence a correspondingly smaller proportion of it enters into the formation of thermodynamically stable HPFs according to the same reactions as when at least 6 atoms F 1 atom P is present.

An excess of HF above that required for satisfying this ratio usually tends to drive the equations toward the right, but when water is present in substantial quantities even greater amounts -of HI must be supplied for best results and to limit addition of a suitable proportion of HF in excess of that required for the corresponding anhydrous reactions causes all the reactions to proceed satisfactorily. This additional HF is effective to counteract the hampering influence of water when the quantity of water in the starting respective equations; if less is supplied but within the limits to'which reference has been made, the reaction proceeds toward the right side of the equation but to a'correspondingly lesser degree.

Again, while water is required in the reaction productas an ionization medium, it may be present as a diluent of the phosphorus-containing inmixture does not exceed 16.25 molecules H20: 1 atom P, and the amount of HF required for this purpose is about that equivalent to the ratio 1 mole HF: 1.625 moles H2O. If in the starting mixture at least this quantity of HP is present in addition .to not less than 6 atoms F: 1 atom P the reactions proceed to equilibrium formation of haveadiscoveredt mor yer that: wh n anhydrous employed, eyen fairly; substantial den;- ciencies; of. below" the said. ratio do& not. prevent entirely equilibrium form-ationofIHBFs, pro: vided the anhydrous-HF is in quantity sufiicient to. satisfy at; least: the. iollowing ratios;

Star-ting- Material Lowest permissible ratio otMolecules anhydrous HIM.

Atom P thestarting mixture,

I H1303. Elmo? H PO4 Hiramv nrolrl Port 1.90 3:13 3,40, o 213 f 1.14 Y on duce equilibrium formation or I-IPFs, but within the limits indicated greater: or lesser amounts are produced depending: upon the proportions of the reactants. A critical composition of a starting mixture in which there is present just sufficient to satisfy the, practical minimum ratio, of 6 atoms F- 1 atom P and to counteract the effect of water is as follows:-

,Gf.=(a+ b) molecules HE: l atomP: (1. 6 25XQ); m lecule H20 in which a is 6' minusthe numberof atoms of fluorine per atom of phosphorus in the phosphorus-containing compound and b may have any value from i) to -10, inclusive, except that for a phosphorus-containingcompound devoid of oxygen, such as PFs, it maynot'beO.

We have stated that some equilibrium formation of HPFs may occur if anhydrous I-IF' onaqueous HF of sufiiciently great concentration be employed in quantities less than those corresponding to the sum or minimum. ratios 6 atoms. 1 atom P- nd 1 me1 HF= 62 m es H2 and thus n ifiei nt even th ore cally to. convert al the hosp o u pr s n to u s that no t q th hi h i on erted t 171 3 6: enter in o the here. a d formation n. eerrespend nc with the 61E 1 1 Bmtioattainment of; quie libr a by th use of Hin.quen iti sinsufficien to satisfy the said ratio is nevertl'reles.S within thescope of our invention.

F pra p e howev r. not le s han 6 atoms F 1 atom P should be present in the: star in mixture. and inaxidifion J1 mole 1.625 moles, 1120 up to a maximum of 16.25 molecules H: 1 atom P to obtain satisfactory yields oi HPFG in equilibrium and we therefore generally prefer to adhere to such proportions and further to utilize more HF than specifically required thereby.

These lowest ratios, r, of molecules anhydrous atom whe e the uil iu mation of a very small quantity of H'PF't wilr just a p ace m y be. exp es d b he followin empiricalequation:

in which a is the number of" oxygen atoms, bthe number of hydrogen atoms andfc the number of fluorine atoms per atom of phosphorus in the starting compound. However. if the compound contains no oxygen- I-EPFe can be produced in accordance; with the invention only if some water is pr nt in; the m t us, fo ample. in the 'presence ofa very smalhquantity of water, event-races of'I-IF added to PF5 will result in the equilibrium formation of HPEs.

'It is usually uneconomical to supply in quantity muchin excess of about 6' F': 1 P beyond any amount. of l-lFrequired to counteract the ef fe o e e p se as t e yields. of HPFG on very; large increases of do notgincrease. corres ondin ly; u chem cally there is. no limitation on the quantity of anhydrous used, s e h e ound th e n w th the os x ess ve. quan ities. of an ydro s no o pou d; eentei nemo e h nt fluorine atomsin each m euli is formed- In other. w rds, we ha e not found a compound, in whi h the ra o. o luori e to pho h us is h her. than. i H238 and an increase in the quantity of HFa-bovethe stated minimum ratios. therefore, may shift the equilibrium to. the right and increase. the yield O but. the-HF nr s u n s, Qfthe re.- t f 1 1 P remains nitse i inal sta epin the final reaction product.

Several reaction mixtures have been tabulated asiel w a one with the c cula d eon.-

eentr ion expec in he p oduct on e s.- snmntion the eact o e to completi n. na. 2' dit ,,.th pe c n PFs fo me int e equilibriuman'd actually found by analysisin the reaction produc s is g n in hese eni co umn and the percent of calcula ed. formed in the last column Thecret- Actual .Percent Equations setting forth proportlonsof constitical' Per- Percent of Calcuuent use cent 'Plft lated. HPEa HPFa PzQs+12EHF 2HPFo.+5 H,20 n 76. 4. 66.2. V 8,6,;7 H POa -l-BHF :2 HPFa-l-IiHiQ- 73.0- 13122- 831-9 67. 0 52. 6 -78 .-5 73 Q 60,5 83.0 80-.-2- 75:0 93.5 89.0, v 86.9 97:6 .89 .4-. 94:9

tain-ing compound to produce HPFs but with As has been pointed out, the use of water-conhigher concentrations at the same ratio there is, as noted, a sharp increase in HPFs yield, while some-equilibrium formation of HPFs may occur at such higher concentrations if the-'HF- supplied taming" HF in most: cases causes equilibrium to De-reached at somewhatlower concentrationsof HPFG! than when anhydrous is: used. When, fori nstanee, 1 mole P205 is reacted with 12 moles is insufiicient to satisfy the 6 F: 1'? ratio. We -a e Q y %-0f thetheoreti equation) cal quantity of HPFs is formed, as compared with 86.7% When anhydrous HF is used (see the first When, however, in the first equation the quantity of HF is substantially increased over that theoretically required, the yield of HPFG is increased; for instance, 1 mole P205 reacted with about 21 moles anhydrous HF gave a product in which 98.9% of the theoretical quantity of HPFs was formed.

As it is advisable to prevent the escape of HF or other gaseous or volatile components, the reaction or succession of reactions therefore is preferably practised under conditions which retain them in the reaction chamber, at least until a condition of substantial equilibrium has been reached, which is usually within one to two hours under laboratory conditions. Thus the reactions may be carried out in an open vessel at reduced temperatures approaching or approximating that of dry ice to produce with suitable agitation substantially the maximum yield of I-IPFs, and even with temperatures as high as C., the yield may be satisfactory in many instances. At still higher temperatures loss of gases and/or vapors cannot readily be prevented unless the reactants are confined in a closed vessel under pressure of the gaseous or volatile substances supplied to or produced by the reaction, and the products cooled at least to room temperature before the vessel is opened. The reactants, or at least one of them, should be introduced in small quantities, preferably during agitation of the mass, so gases and vapors may be more readily retained and either batch or continuous methods of operation afford good results provided the conditions stated be substantially observed. Of course in view of the nature of HF, the equipment should be adequately corrosion resistant and therefore made of or plated with platinum or silver, or be of special stainless steel or other suitable metal, or its surfaces should be coated with a resin or other substance not readily attacked by HF.

More specifically, procedures appropriate for carrying out the several reactions are as follows:

(a) For the reaction of phosphorus pentoxide, with anhydrous HF at the temperature of dry ice, small quantities of P205 are added slowly during stirring of the HF and preferably in total amount insuflicient to theoretically satisfy all the latter, so that after completion of the reaction an excess of HF remains in the mass, which, after substantial equilibrium has been reached, is allowed to warm gradually to room temperature. If the P205 is placed in a pressure-tight vessel and anhydrous HF injected thereinto during mechanical agitation, coolin is unnecessary, and if sufiicient HF is supplied to insure an excess over that theoretically required to react with all the P205 for the production of HPFs, maximum yield of the latter is attained. Inasmuch as appreciable heat is evolved during the reaction the vessel should not be openeduntil after equilibrium has been reached and room temperature restored.

(b) As set forth in the second equation, HPO3 acid contains other condensed phosphoric acids and orthophosphoric acid and may be utilized in the preparation of hexa acid. The equation or thereabcuts, and again with HF in excess of the theoretically required quantity for reacting with all the phosphorus containing constituents. These reactions may also occur as intermediate ones in connection with the production of HPFe from P205 and/or its hydration products since an initial reaction may result in creation of at least some HzPOsF and/or HPOzFz which then reacts with additional HF.

(f) POFs, being a gas at normal temperature and pressure, may be passed into liquid anhydrous HF cooled by dry ice, preferably until sufficient POFs has been dissolved in the liquid to react with all but a little of the anhydrous HF; attainment of substantial equilibrium usuallyrequires several hours and the gas is therefore passed slowly into the HF to prevent wasting it.

POF3 may be obtained from any available source or may be producted by methods disclosed in the literature. Of these We prefer to employ the reaction between calcium fluoride (CaFz) and phosphorus pentoxide (P205) as described by Tarbutton, Egan and Frary in the Journal of the American Chemical Society, vol. 63, page 1782 (1941).

(g) A similar procedure may be followed for the reaction of HF with gaseous PF5, indicated in the last equation of the tabulation, although, if desired, temperatures up to -10 C. may be used in this case, and as has been noted, it is essential that at least a small amount of water be present to afford an ionization medium for the HPFG produced. The gaseous PF5 may also be forced with pressure into a closed vessel containing the hydrofluoric acid, at temperatures substantially above 10 C.

Production of HPFs by the reaction of HF with PF5 is claimed in our copending divisional application, Serial No. 789,758, filed December 4, 1947, entitled Method of producing hexafluorophosphoric acid.

The reactions of the several tabulated equations thus produce high concentration hexa acids when a minimum quantity of water, or none at all, is present at the start of the reaction and correspondingly lower concentrations of HPFs as the amount of water present at the start approaches the maximum which is permissible.

For the best production of HPFs, we therefore .prefer to limit the amount of water present at 11 .tration, .crystallizationmay begin just below 315 C., the melting point of hexahydrate crystals or at a lower temperature, and .continues during further reduction of temperature. As the crystals sometimes form relatively slowly, reduction in temperature should progress gradually and .the hard coarse crystals of the hexahydrate may readily be separated from the surrounding liquid .by centrifuging, filtration, pressing between lay- :ers of a water-free absorbent material like well- .dried filter paper or by any other .suitable mechanical means. During the separation the crystals should be kept at a temperature below their melting point (about 31.5" C.'), and even after separation their exposure to temperatures 'not far from their melting point, and especially to conditions of high relative humidity, should be avoided since they are very deliquescent and :so tend to absorb moisture rapidly from the air and return to solution.

Even if the HPFs solution contains up to about I .10 molecules of H20 per-molecule of HPFG, corresponding to about 44.8% HPFG, the hexahydrate may be crystallized readily therefrom within'the temperature range 31;5 C.'to 35 C., and

still more dilute solutionstmay'under certain circumstances yield small quantitiesof hexahydrate crystals at extremely low temperatures. But when crystallized hexahydrate is to be recovered from very dilute solutions, we prefer to use a preliminary concentrating step, introducing the solution into a high'vacuum at temperatures below 30 C. either in a thin film or in any other formoffering evaporating surfaces constantly renewed, as by spraying into .an evacuated vessel or the like.

' Crystallization of the hexahydrate may be considered-either as a step in :a methodof producing this substance or as one for purifying a beam acid containing, in addition to the HPFS, various other substances, *suchas foreign matter, intermediate products of reaction, -or even remain ng .unreacted portions of the initial reactants, since pure hexa acid solutions' 'may readily be reconstituted by dissolving "hexa'hydra'te crystals in water. For example when P205 is employed and 'the HF supplied converts "less than all the phosphorus'present to'I-IPFe, the solution may contain after equilibrium has been established minor quantities of Hz'PozFand/or'HPozFz, and unreacted HF. Hexahydrate crystals may, however, form in the reaction product=on suitable cooling, and may easily-be separated from the liquidcontaming such other substances, and then be dissolved in water to produce a pure 'hexa acid.

HPFE+wateriHPozFz-l-hydrofiuoric acid For-producing pure salts 'thru the :use of the Thexahydrate, "we therefore prefer 'to "neutralize the hexahydrate crystals with metal oxides, hydroxides, etc., or to use the melted .hexa'hvdrate or its aqueous solution without allowing them to stand rafter melting. We :have observed that with :increasingdilution LOlf 'theihexahydrate with 12 water (and the .sameis true .for any hexa acid preparation), .the establishment .of an equilibrium, resulting in hydrolysis-of .some HPFe, he-

.comes slower and slower until at concentrations .of only a few percent ,l-IPF the .acidseems to .be perfectly stable and does not seem .to establish zui equilibrium.

This, however, is not true. .Such a high .dilu tion, if not in equilibrium, is thermodynamically unstable. Much time .may be required for the equilibrium to establish itself, but heating .under closed-container conditions, followed by cooling establishes this equilibrium .at .once. The .described facts are also the reason why the freak HPFs, formed in traces in the reaction of P205 with aqueous- HF in proportions beyond the, limits prescribed in our .invention, .can exist .for a certain time while it is actually thermodynamically unstable.

A hexa acid prepared in accordance with -.0'l11' disclosures is a non-oxidizingagent which maybe used advantageously in organic reactions as a condensing or polymerizing or esterifying catalyst, Hexa acid, in the form of an equilibrium product or after isolation as the hexahydrate, is useful also .as an electrolyte. The salts of .the hexa acid are made readily through neutralization of the acid with metaloxides, hydroxides,.carbonates, etc. These salts are applicable, among other purposes, in electroplating, in electrodepositionrof metals from electrolytic baths and for .other electrochemical work. Organic hexafluorophosphates can be prepared and may .find general application in the manufacture oi various organic compounds; when they are utilized :in processes similar to those in which the corresponding perchlorates have been employed or wouldhave been employed, if the perchlorates had been less dangerous, the danger of explosion inherent in use -01 .organicperchloratesdoes not=exist since the hexafillorophosphates are apparently non-explosive. The aromatic diazonium salts, possessing wery limited solubility in waterandexcellentstability in dry form are convenient sources of 'PFaaga which is itself a useful polymerization catalyst.

We shall now set forth more Specifically in :several illustrative instances the mannerinwhich our invention has been employed for the :produc'- tion of useful substances without, :however, :intending thereby to limit orconfineiit thereto.

Example 1.--.48.9 parts of "P205 were added slowly in small portions to 182.5 .parts of liquid anhydrous HF, in a ratio'of '121moles HF lmole P205, cooled with dry ice in a platinum bottle and constantly stirred during the addition of the P205; after gradual warming to room temperature themixturewas found'to contain66.2% HPFe :or approximately 86.7% of the amount theoretically obtainable from said quantities of the ingredients .reacting in accordance with the equation:

65.7 parts of the reaction product were .cooled .in ice and 18.5 parts .of water .of.;.ice -temperature added. The resulting mixture was ,-kept forone day at 10 .C. during which aicrystalli-ne precipitate of 65.4 parts of .hexah-ydrate was formed, ac-

, counting for 86.3% of the HPFs-orlginallygpresent in the reaction product and thus recovered as crystalline hexahydrate.

Example 2.-Anhydrous HF and P205 l'eacted under conditions similar :to those 'in :Example .11 but in the proportion of I1=5partsanhydrousHF and 2.4 parts BzOainla-ratio Qfit211531101fi3 HP: 1

mole P205, produced a solution containing 51.1% HPFo or 98.9% of the quantity theoretically obtainable from reaction of thestated quantities of the reactants.

Example 3.-36.8 parts of HPO3 mixed slowly at ice temperature with 55.8 parts of anhydrous HF, in a ratio of 6.064 moles HF 1 mole HPOs, were agitated mechanically for one day in a closed platinum bottle maintained at a temperature of 10 C. The resulting solution on analysis was found to contain 60.9% I-IPFs or 83.9% of the quantity theoretically obtainable from the initial amounts of the ingredients reacting in accordance with the equation:

Example 4.50 parts of liquid anhydrous HF in a platinum bottle was cooled with brine at -10 C.; water-free H3P04 was slowly added to the HF while the latter was being stirred, until 40 parts in all had been supplied, in a ratio of 6.124 moles HF 1 mole H3PO4. The whole mixture was then allowed to warm gradually to room temperature, a portion of the product analyzed and the HPFs content determined with nitron (C20H16N4) In accordance with the equation, HsPO4+6HF::I-IPFs+4l-IzO, a solution containing 66.2% HPFs theoretically should have been produced; analysis showed 52.0% HPFs actually present, indicating an actual yield of 78.5% of the theoretical one.

A quantity of the solution was set aside for one day and maintained at temperature of 15 0.: a cake formed consisting of coarse grainy crystals which after separation from the liquid and drying between layers of thoroughly pre-dried filter paper were found to consist of substantially pure hexahydrate in a yield of 60.7 parts and to contain 74.5% of the HPFe originally in the solution. The remaining liquid contained but relatively little water inasmuch as originally there was not enough to supply water of crystallization for all the HPFs, thus indicating that a larger yield of hexahydrate might have been obtained had more water been present. The crystals on analysis were found to contain 57.38% HPFs, as compared with 57.46%, the theoretical value of HPFc in the hexahydrate.

Example 5.-32.73 parts of commercial tetraphosphoric acid containing 36.2% was mixed with 45.86 parts of liquid anhydrous HF at ice temperature and resulted in a solution which contained 58.8% HPFs, corresponding to a yield of 82.9% of the quantity theoretically obtainable.

Example 6.--21.9 parts of liquid anhydrous HF in a platinum bottle cooled with brine at20 C. was stirred during addition in small quantities of 21.6 parts of pur commercial HzPOsF, in a ratio of 5.069 moles HF 1 mole HzPOaF. After gradual warming to room temperature the resultant solution was analyzed and found to contain 60.1% HPFs, corresponding to a yield of 83.0% of the quantity theoretically produced in accord ance with the equation:

Example 7.-39.1 parts of liquid anhydrous HF in a platinum bottle cooled with brine at 20 C. was stirred during addition in small quantities of 49 parts of pure commercial HPO2F2, in a ratio of 4.069 moles HF 1 mole HPOzFz. After gradual warming to room temperature the resultant solution contained 74.4% HPFs, corresponding to phosphorus 14 a yield of 93.5% of the quantity theoretically produced in accordance with the equation:

Example 8.-5.1 parts of liquid anhydrous HF in a platinum bottle was cooled with dry ice while 8.3 parts of gaseous POFs was passed into the liquid in a ratio of 3.194 moles HF: 1 mole POFa. After cooling overnight in dry ice the mixture was slowly warmed to room temperature and found on analysis to contain 84.9% HPF6, corresponding to a yield of 97.6% of theo retical value in accordance with the equation:

Example 9.-1.9 parts of a 52.6% HF solution was cooled in a platinum bottle to 10 C. and 6.3 parts of gaseous PF5 passed into the liquid, in a ratio of 1 mole PFs 1 mole HF: 1 mole H2O. The resulting solution contained 84.5% HPFs corresponding to a yield of 94.9% of the theoretical yield.

Example 10. A quantity of a solution, obtained by the reaction of P205 with a 67.2% HF solution in a ratio of 1 mole P205 12.34 moles HF: 6.684 moles H20 and containing 27.6% HPFs, was cooled to 15 C., maintained at the temperature for 12 hours and then further cooled to 10 C. and maintained at that temperature during the subsequent 12 hours. The crystalline precipitate of hcxahydrate produced was analyzed quantitatively and found to contain 75.5% of the HPFG content of the original solution. 15 parts of the crystalline hexahydrate dissolved in an excess of ice-cooled aqueous ammonium (25% NI-Is) was evaporated in a silver dish on a steam bath to just short of dryness, the residue then dried on filter paper in the air, redissolved in hot water, filtered, made slightly ammoniacal and the steam bath evaporation repeated. After final drying on filter paper the second residue contained 8.7 parts of pure ammonium hexafluorophosphate, indicating a yield of 90.4%.

Example 11.To 7.9 parts of potassium hydroxide (KOH) dissolved in 24 parts of water and cooled in a salt-ice mixture, 14.6 parts of a 74.4% hexa acid was gradually added during agitation of the KOH solution. A precipitate, gel-like in appearance but actually well crystallized, formed immediately and when removed by filtration, pressed between layers of absorbent paper and finally dried on a porous plate, was found to consist of 8.2 parts of relatively pure potassium hexafluorophosphate which recrystal lized readily from boiling water made slightly alkaline with KOH and was thus recovered in an even more highly purified state.

Example 12.To illustrate further the use of our I-IPFs solution we mixed 9.3 parts of aniline, 20 parts of water and 24.7 parts of 37% hydrochloric acid, keeping the mixture at ice temperature and agitating it during further slow addition of 6.9 parts of sodium nitrite dissolved in 20 parts of water. To the clear phenyl diazonium chloride solution obtained, 23 parts of a 74.4% HPFs solution was added, accompanied by instantaneous formation of a paste of fine crystals which was filtered on a Buechner funnel with suction, washed with 40 parts of ice water, then with ethyl alcohol, finally with ether,

and thereafter dried on a porous plate. Analysis showed a yield of 20.3 parts of phenyl diazonium hexafluorophosphate which after drying in a desiccator over P205 was thermally decomposed into liquid phenylfiuoride (CsHsF) and gaseous PFB.

It will be understood alt the foregoing examples" are for purposes of illustration only and that they are not intended as restrictive of the manner in which either the hexa acidsolutions or the heXahyd-rate in crystalline form may be produced, of the objects of. such production or' the usefulness of the products.

It will therefore be understood that While we:

have herein described in accordance with. the statute what we now consider the preferred procedure for practising the invention we recog-- nize that many modifications and variations, both chemical and physical, will occur to those skilled in the art and may be employed, if desired, without departing from the spirit and scope of the invention as defined in the: appended claims.

Having thus described our invention, we claim and desire to protect by Letters Patent of the United States:

1 The method of producing hexafluorophos-- phoric acid by equilibrium formation which comprises reacting phosphorus oxytritluoride With hydrofluoric acid in the ratioof about 3 moles of hydrogen fluoride per mole of phosphorus oxytrifluoride in a mixture containing not less than one additional molecule of hydrogen fluoride per 1,625 molecules of uncombined Water present in the original reaction mixture and not over 6.25 molecules of uncornbined water per atom of phosphorus, preventingthe escape; of gaseous compounds during the reaction and carrying WILLY LANG-1i},w RALPH LIVINGSTON.

REFERENCES CITED The following: references are of recordin the? fi-l-e of this patent:

UNITED STATES PATENTS Name Number Date OTHER" REFERENCES Technical Service. Bulletin EPA-1, Monofluoro phosphoric Acid, Difiuoro Phosphoric-Acid, page 3, Copyright 1-944, Ozark Chemical 00., Tulsa, Oklahoma.

Lange Oct. 8,1945" 

